1. Field of the Invention
The present invention generally relates to monitoring electrical grounding, and more particularly to a device and system that automatically monitors an operator's grounding mechanisms only when the operator is actually present in front of a work station.
2. The Related Art
How to prevent electro-static discharge (ESD) from damaging valuable equipment or causing critical fabrication process to fail is still an important issue in high-tech industries. It is well known that proper grounding is the essential solution. However, despite the advancement of technology, ensuring such a proper grounding is not as easy as most people imagine.
A typical manufacturing environment usually contains a number of assembly lines, and each assembly line usually contains a number of work stations, each for a specific assembly task or manufacturing operation. To prevent ESD from damaging the parts, devices, or the semi-products being assembled, the operator at the work station is usually required to wear an anti-static wrist strap, the floor is usually paved with an anti-static floor mat, and the table top of the work station is usually covered with an anti-static table mat. As illustrated in FIG. 1a, the floor mat 10, table mat 20, and the wrist strap 30 are usually electrically connected to a common-point ground 40 of the work station by grounding cables, respectively (for simplicity, the drawing only shows a ground cable connecting the wrist strap 30 and the common-point ground 40). The common-point ground 40 is usually a metallic plate fixedly positioned at some place of the work station with a plastic cover for protection. The common-point grounds of an assembly line's work stations are series- or parallel-connected together, which are in turn connected to an equipment ground or an earth ground of the manufacturing facility (again, for simplicity, the equipment and earth grounds are not shown in the drawing). As such, the static electricity carried by or accumulated on an operator sitting or standing in front of the work station is discharged to the earth through the wrist strap, table mat, or the floor mat, via the common-point ground of the work station and then the equipment or earth ground of the manufacturing facility, thereby preventing potential hazards from ESD.
The aforementioned grounding system is a proven solution and has been widely adopted for years. However, it suffers a number of disadvantages. First, this grounding system works only if the wrist strap, the floor mat, and the table mat are properly connected to the common-point ground. However, the grounding cables therebetween could be rusted or broken, or the grounding cables could be disconnected from the common-point ground due to the movement of the operator. In addition, when the operator has to take a break or to go for lunch, he or she may take down the wrist strap and leave it on the work station. Or, in most of the existing implementations, the grounding cable of the wrist strap has a plug at one end so as to plug into a socket of the common-point ground. Therefore, the operator unplugs the grounding cable (but still wears the wrist strap) before going for a break or lunch. When the operator returns, he or she then put the wrist strap back or plug the grounding cable again. As can be imagined, a lazy operator may avoid wearing the wrist strap; or an absent-minded operator may forget to put back or re-plug the wrist strap after returning to his or her post. The static electricity carried by or accumulated on the operator cannot be discharged to the ground, and may very possible damage the valuable equipment or parts or semi-product or completed product at the work station.
As such, the present inventor has disclosed a wireless monitoring device for a work station operator to see if the operator has properly worn a wrist strap or similar grounding mechanism (U.S. patent application Ser. No. 11/964,151 filed on Dec. 26, 2007, hereinafter, the previous application). The representative drawing of the previous application is attached as FIG. 1b. As illustrated, the monitoring device 100 mainly contains a microprocessor circuit 200 as its core. The device 100 is connected to the mains via a power cable or via an external power supply (e.g., a power adaptor such as those used by a notebook computer). The connection to the mains is very important in that, on one hand, the electricity extracted from the mains is processed by a power unit 500 of the device 100 to provide appropriate direct-current (DC) voltages to the microprocessor circuit 200. On the other hand, the ground 60 of the mains is thereby electrically introduced into the device 100. The device 100 is also connected to the manufacturing facility's equipment ground or earth ground 50 (hereinafter, jointly referred to as earth ground) via an interface 120. This can be achieved by connecting a common-point ground 40 of the work station or, as illustrated, by directly connecting the earth ground 50. Additionally, the device 100 is connected to two conducting wires 31, 32 of a wrist strap 30 via another interface 110. As illustrated, an end of the wire 31 is electrically connected to the earth ground 50 inside the device 100 whereas an end of the wire 32 is electrically connected to the mains ground 60 via the microprocessor circuit 200. The other ends of the wires 31, 32 are connected to two conducting plates embedded in an insulating casing of the wrist strap 30, respectively. The conducting plates 33 are usually exposed from the inside of the insulating casing so as to contact an operator's wrist skin 70. As such, when the operator has properly worn the wrist strap 30, a discharge circuit, shown by the dashed lines of FIG. 1b, is established from the mains ground 60, through the earth, the earth ground 50, the wire 31, the skin 70, the wire 32, and then to the mains ground 60 via the microprocessor circuit 200. A major function of the microprocessor circuit 200 is in determining if the discharge circuit has an appropriate resistance. The rest of the details could be found in the previous application and is omitted here.
There are still some disadvantages for the monitoring device 100 taught by the previous application. First of all, the monitoring device 100 is applicable to grounding mechanism with two conducting wires, yet there are quite some commercially available single-wire grounding mechanisms. The monitoring device 100 is therefore not applicable to these single-wire grounding mechanisms. Secondly, the monitoring device 100 could only monitor the grounding mechanisms an operator is equipped with but cannot provide additional ESD protection to the operator. For example, if the connection between the common point ground 40 and the earth ground 50 is somehow disconnected (e.g., at a place marked by X in FIG. 1b), the monitoring device 100 would signal an alarm and the operator has to stop his or her work until the disconnection is fixed. But, as can be seen in FIG. 1b, the mains ground 60 connected to the monitoring device 100 could provide the required grounding and the operator actually does not need to stop his or her work. In other words, the monitoring device 100 could actually provide the mains ground 60 as an auxiliary grounding for additional ESD protection. Additionally, according to the previous application, every monitoring device 100 would require a power cable or a power adaptor, and an outlet for plugging the power cable or the power adaptor, which would increase the product and installation costs.